The present invention relates to a magnet apparatus and a magnetic resonance imaging (MRI) apparatus. More particularly, the present invention is concerned with a magnet apparatus having gradient coils disposed in a bore of a cylindrical superconducting magnet, and an MRI apparatus employing the magnet apparatus.
MRI apparatuses are designed to reconstruct images according to magnetic resonance signals acquired through a magnet apparatus. A superconducting magnet apparatus has gradient coils and radiofrequency (RF) coils disposed in a bore of a cylindrical superconducting magnet.
In order to suppress eddy currents induced in a metallic housing that is part of the superconducting magnet, shield coils are interposed between the internal surface of the metallic housing and the external surface of the gradient coils in order to produce magnetic fields whose directions are opposite to the directions of magnetic field gradients acting on the metallic housing (refer to, for example, Patent Document 1).
[Patent Document 1] Japanese Unexamined Patent Publication No. 2000-157515 (p. 4 and 5, FIGS. 1 and 2)
The inclusion of shield coils brings about as a side effect degradation of the efficiency in using power for production of magnetic field gradients. Therefore, a greater power is needed for production of desired magnetic field gradients than when the shield coils are not included. Moreover, heat dissipation increases, and vibrations and noises derived from production of magnetic field gradients grow in intensity.